1. Field of the Invention
The present invention relates to displays, such as organic electroluminescence (EL) displays, in which pixel circuits, each having an electro-optical element of which luminance is controlled based on a current value, are arranged in a matrix, and particularly to so-called active-matrix displays in which the value of the current flowing through an electro-optical element is controlled by insulated-gate field effect transistors provided in each pixel circuit.
2. Description of the Related Art
In an image display, e.g., in a liquid crystal display, a large number of pixels are arranged in a matrix, and the light intensity is controlled on each pixel basis in accordance with information on an image to be displayed, to thereby display the image.
This pixel-by-pixel control is similarly implemented in an organic EL display and the like. The organic EL display has a light-emitting element in each pixel circuit, and therefore is a so-called self-luminous display. The organic EL display has the following advantages over the liquid crystal display: higher image visibility, no necessity for a backlight, and higher response speed.
Furthermore, the organic EL display is greatly different from the liquid crystal display and the like, in that a color grayscale is obtained through control of the luminance of each light-emitting element based on the value of the current flowing through the light-emitting element, i.e., the light-emitting elements are current-control elements.
The kinds of drive systems for the organic EL display include a simple-matrix system and an active-matrix system similar to the liquid crystal display. The simple-matrix system has a simpler configuration but involves problems such as a difficulty in the realization of a large-size, high-definition display. Therefore, currently, the active-matrix system is being developed more actively. In the active-matrix system, the current that flows through a light-emitting element in each pixel circuit is controlled by active elements, typically by thin film transistors (TFTs), provided in the pixel circuit.
FIG. 1 is a block diagram showing the configuration of a typical organic EL display.
As shown in FIG. 1, a display 1 includes a pixel array part 2 in which pixel circuits (PXLC) 2a are arranged in an m×n matrix, a horizontal selector (HSEL) 3, and a write scanner (WSCN) 4. Furthermore, the display 1 includes data lines DTL1 to DTLn that are selected by the horizontal selector 3 and supplied with data signals in accordance with luminance information, and scan lines WSL1 to WSLm that are selected and driven by the write scanner 4.
The horizontal selector 3 and the write scanner 4 are formed on polycrystalline silicon in some cases, and are formed in the periphery of pixels as MOSICs or the like in other cases.
FIG. 2 is a circuit diagram showing one configuration example of the pixel circuit 2a of FIG. 1 (refer to e.g. U.S. Pat. No. 5,684,365 and Japanese Patent Laid-Open No. 8-234683).
The pixel circuit of FIG. 2 has the simplest circuit configuration among a large number of proposed circuits, and is based on a so-called two-transistor drive system.
The pixel circuit 2a of FIG. 2 includes a p-channel thin-film field effect transistor (hereinafter, referred to as a TFT) 11, a p-channel TFT 12, a capacitor C11, and an organic EL element (OLED) 13 as a light-emitting element. Furthermore, in FIG. 2, DTL and WSL denote a data line and a scan line, respectively.
The organic EL element has a rectification function in many cases, and therefore, is often referred to as an OLED (Organic Light Emitting Diode). Although a diode symbol is used for representation of a light-emitting element in FIG. 2 and other drawings, the OLED in the following description does not necessarily need to have a rectification function.
In FIG. 2, the source of the TFT 11 is connected to a supply potential Vcc, and the cathode of the light-emitting element 13 is connected to a ground potential GND. The pixel circuit 2a of FIG. 2 operates as follows.
Step ST1:
When the scan line WSL is turned to the selected state (to a low level, in this example) and a writing potential Vdata is applied to the data line DTL, the TFT 12 conducts, and thus, the capacitor C11 is charged or discharged, so that the gate potential of the TFT 11 becomes Vdata.
Step ST2:
When the scan line WSL is turned to the non-selected state (to a high level, in this example), the data line DTL is electrically isolated from the TFT 11. However, the gate potential of the TFT 11 is stably held by the capacitor C11.
Step ST3:
The current that flows through the TFT 11 and the light-emitting element 13 has a current value dependent upon the voltage Vgs between the gate and source of the TFT 11, and the light-emitting element 13 continues to emit light with luminance dependent upon this current value.
Hereinafter, the operation of selecting the scan line WSL to thereby transmit luminance information supplied to the data line to the inside of a pixel, like that of the step ST1, will be expressed by using a verb “write”.
In the pixel circuit 2a of FIG. 2, after the potential Vdata is written, the light-emitting element 13 continues to emit light with constant luminance until the next rewriting of the potential.
As described above, in the pixel circuit 2a, the voltage applied to the gate of the TFT 11 as a drive transistor is varied to control the value of the current flowing through the EL light-emitting element 13.
Because the source of the p-channel drive transistor is connected to the supply potential Vcc, the TFT 11 typically operates in the saturation region. Therefore, the TFT 11 serves as a constant current source for a current having a value represented by Equation (1).
(Equation 1)Ids=½·μ(W/L)Cox(Vgs−|Vth|)2  (1)
In Equation (1), μ denotes the carrier mobility, Cox denotes the gate capacitance per unit area, and W and L denote the gate width and gate length, respectively. In addition, Vgs denotes the voltage between the gate and source of the TFT 11, and Vth denotes the threshold voltage of the TFT 11.
In a simple-matrix image display, each light-emitting element emits light only at the moment of being selected. In contrast, in the active-matrix system, each light-emitting element also continues to emit light after completion of writing as described above. Therefore, the active-matrix system is advantageous in driving a large-size and high-definition display in particular, because the active-matrix system can decrease the peak luminance and peak current of the light-emitting elements compared with the simple-matrix system.
FIG. 3 is a diagram showing a change of the current-voltage (I-V) characteristic of an organic EL element over time. In FIG. 3, the full-line curve indicates the characteristic of the initial state, while the dashed-line curve indicates the characteristic after the change over time.
In general, the I-V characteristic of an organic EL element deteriorates with elapse of time as shown in FIG. 3.
However, the two-transistor driving of FIG. 2 is constant-current driving, and therefore, a constant current continues to flow through the organic EL element, as described above. Thus, even when the I-V characteristic of the organic EL element deteriorates, the light-emission luminance thereof does not change over time.
The pixel circuit 2a of FIG. 2 is formed of p-channel TFTs. If the pixel circuit 2a can be formed of n-channel TFTs, an existing amorphous silicon (a-Si) process can be used for TFT fabrication. This can reduce the cost of the TFT substrate.
A description will be made below about a basic pixel circuit obtained by replacing the transistors by n-channel TFTs.
FIG. 4 is a circuit diagram showing the pixel circuit obtained by replacing the p-channel TFTs in the circuit of FIG. 2 by n-channel TFTs.
A pixel circuit 2b of FIG. 4 includes n-channel TFTs 21 and 22, a capacitor C21, and an organic EL element (OLED) 23 as a light-emitting element. Furthermore, in FIG. 4, DTL and WSL denote a data line and a scan line, respectively.
In this pixel circuit 2b, the drain side of the TFT 21 as a drive transistor is connected to a supply potential Vcc, and the source thereof is connected to the anode of the EL element 23, so that a source follower circuit is formed.
FIG. 5 is a diagram showing the operating point of the TFT 21 as the drive transistor and the EL element 23 in the initial state. In FIG. 5, the abscissa indicates the voltage Vds between the drain and source of the TFT 21, while the ordinate indicates the current Ids between the drain and source of the TFT 21.
As shown in FIG. 5, the source voltage is determined by the operating point of the TFT 21 as the drive transistor and the EL element 23, and differs depending on the gate voltage.
Because the TFT 21 is driven in the saturation region, the TFT 21 outputs the current Ids with a current value in accordance with Equation (1), derived from the voltage Vgs corresponding to the source voltage of the operating point.